An organic light emitting diode, briefly OLED (short form for the English expression “organic light emitting diode”) is a luminous component made from organic semi-conducting materials. The OLED technology is intended for visual display application (e.g. television screen, PC screen) among others. Another field of application is represented by large scale space illumination as well as advertising illumination). Considering the materials available the employment of OLEDs as area light sources, flexible display and electronic paper (E-paper) will only be a matter of time.
An OLED design consists of a plurality of thin organic layers. Usually a hole transport layer (HTL) is thereby applied onto an anode (e.g. indium tin oxide, ITO) partly or completely transmissive to light, which is located on a transparent substrate such as a sheet of glass or a transparent layer of plastic material such as polyethylene terephthalate (PET). Depending on the method of production, between the anode and the hole transport layer an additional layer of PE-DOT/PSS (poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate, Baytron P) is often applied which serves for lowering the injection barrier for holes and furthermore smoothens the surface. Onto the hole transport layer, a layer is applied which either contains the dye (app. 5-10%) or—rarely—completely consists of the dye (e.g. aluminum tris(8-hydroxy quinoline)=Alq3). This layer is referred to as an emitter layer (EL). In some cases an electron transport layer (ETL) is applied thereupon. Finally a cathode (consisting of a metal or an alloy having low electron exit work, e.g. calcium, aluminum, magnesium/silver alloy) is applied by vapour deposition under high vacuum. For lowering the injection barrier for electrons a very thin layer of e.g. LiF or CsF is applied by vapour deposition between cathode and ETL or emitter. For completion the electrode may be coated with silver or aluminum for protective reasons. However, the transparent layer can likewise be adjacent to the cathode. Then the cathode is partly or completely transmissive to light.
An organic solar cell (OSC) is a photovoltaically active component made from organic semi-conducting materials or a mixture of organic and inorganic semi-conducting materials. These components are prospectively considered as inexpensive alternatives for inorganic solar cell materials.
An OSC design consists of a plurality of thin organic or in part even of inorganic layers. Usually a photovoltaically active layer is thereby applied onto an electrode (e.g. indium tin oxide, ITO) partly or completely transmissive to light which is located on a transparent substrate such as for example on a sheet of glass or a transparent layer of plastic material such as polyethylene terephthalte (PET). Depending on the method of production, between the electrode and the photovoltaically active layer an additional layer of PE-DOT/PSS (poly 3,4-ethylene dioxythiophene)/polystyrene sulfonate, Baytron P) is often applied which serves for adapting energy levels between the transparent electrode and the photovoltaically active layer, and furthermore smoothens the surface. The photovoltaically active layer either is a mixture of a donor and acceptor compound, or a sequence of layers of donor and acceptor compounds. One common example of a donor compound is poly-3-hexylthiophene (P3HT); the acceptor compound is exemplified by phenyl-C61-butylic acid methyl ester (PCBM). Additional interlayers may be present. Finally an electrode consisting of a metal or an alloy having low electron exit work (e.g. calcium, aluminum, magnesium/silver alloy) is vapour deposited under high vacuum. For completion the electrode may be coated with silver or aluminum for protective reasons.
Methods of production of such electronic components are known from the references U.S. Pat. No. 7,041,608, U.S. Pat. No. 6,593,690 as well as U.S. Pat. No. 7,018,713. Therefrom, electrostatic spraying methods for application of layers are known among others. From this state of the art it will not be realized that spraying under certain circumstances may lead to practically manufacturable electronic components enabling an extremely good out-coupling efficiency.
For creation of light a voltage is applied to the electrodes. The electrons are now injected by the cathode whereas the anode provides the holes. Hole (=positive charge) and electron (=negative charge) are drifting towards each other and ideally meet in the emitter layer, and this is why this layer is also referred to as recombination layer. The electrons and holes form a bound state which is referred to as exciton. Depending on the mechanism the exciton already represents the excited state of the dye molecule, or the decay of the exciton will provide the energy for the excitement of the dye molecule. This dye exhibits various states of excitement. The excited state can return to the ground state, thereby emitting a photon (particle of light). The colour of the emitted light depends on the energy difference between excided state and ground state and may selectively be changed by variation of the dye molecules. It is the distance between the so called HOMO and LUMO which is responsible for the wavelength of light. With the inorganic semiconductor, HOMO and LUMO correspond to the valence band and conduction band.
Disadvantageously, because of internal total reflexion, the created light will exit the glass sheet predominantly laterally and only a minor part of it will exit the glass sheet or the plastic layer frontwardly. Since usually exclusively the light which is emitted frontwardly is suitable only a small part of the created light will actually be used.
In order to improve the out-coupling of usable light, according to EP 1 100 129 A2 the provision of an interlayer having a low refractive index, consisting of aerogel, which is located between the transparent substrate and the transparent electrode is proposed. It is said that the light yield may thus be doubled. However, an interlayer consisting of aerogel cannot be manufactured practically and hence this proposal has not been put into practice.
According to WO 2005/109539 A1, in order to be able to out-couple light more practically the provision of a light emitting diode of the previously mentioned type with a porous buffer layer having a low refractive index being located between the transparent electrode and a layer consisting of an organic semiconductor is proposed. The size of the pores is in the nanometer range. In order to be able to apply an organic semiconductor layer the buffer layer is said to be of closed pores. The porous buffer layer is produced either by pyrogenes which will be eliminated from the buffer layer after layer formation, or by foaming. The porous buffer layer is said to consist of a hole conducting material.
In practice it was accomplished to produce the buffer layer known from WO 2005/109539 A1, having a refractive index of 1.6 (wavelength-dependent) and thus improve the light yield. However, the improvement is minor. Buffer layers having a lower refractive index inferior to 1.6 to further increase the light yield could not be realized. Furthermore a porous buffer layer was not realized successfully either in a practice-oriented way. For example pores cannot be manufactured by foaming in a practice-oriented way since a foaming process basically results in uneven layer thicknesses. Thus, desired layer thicknesses of the buffer layer could not be manufactured consistently. The creation of porosity with the aid of pyrogenes did not result in pores having suitable pore sizes.